1. Field of the Invention
The invention relates to a downhole drilling coupling and more particularly to a rotor-to-rotor coupling for synchronizing two positive displacement motors.
2. Background of the Invention
In the oil and gas industry, downhole drilling motors have been used for many years, especially in directional drilling. Typically, the drilling motor is contained in a housing, and supplies a rotational force by way of a drive shaft for turning a drill bit. The motor housing is connected to the lower end of a conventional drill string comprised of sections of drill pipe and drill collars. The drill string extends to the surface, where it is connected to a kelly, mounted in the rotary table of a drilling rig.
As the drilling begins, drilling fluid is pumped down through the drill string to the bottom of the bore hole and back up to remove the cuttings resulting from the drilling operation. Since the drilling fluid is constantly flowing through the drill string, a positive displacement motor is commonly used.
Positive displacement motors, or Moineau motors, have fixed volumetric displacement and, thus, their speed is directly proportional to the flow rate. These types of motors are well known in the art. The positive displacement motor (PDM) comprises a helical rotor within a helical cavity of a stator. The stator is bonded to the housing of the motor and is composed of rubber or rubber-like helical lobes, or threads for coacting with the helical rotor. The number of stator lobes is one more than the number of rotor lobes, thus creating a number of chambers along the length of the stator. As the drilling fluid is pumped down the drill string, under pressure, and through the cavity of the stator, the fluid imparts a torque to the rotor, causing the rotor to turn and pass the fluid from chamber to chamber. As the helical rotor rotates, it also orbits eccentrically about the axis of the stator in a reverse direction relative to its axial rotation. The rotor torque is then transferred to the drive shaft of the drill bit by a universal joint.
The torque developed by the PDM is also well understood to be proportional to the rotor pitch (or distance the fluid advances in one rotation of the rotor) and the number of rotor and stator lobes. While in theory, one long motor section provides a large amount of torque, in practice, single long sections have not been used with much success. Manufacturing, transportation and handling practicalities limit the length. Further, if any part of the motor malfunctions, the entire motor must be removed and replaced. This is particularly true of the stator. As the stator must have an effective hydraulic seal around the rotor, it is a vulnerable portion of the motor because it is subjected to continuous rubbing and deformation by the rotor. Additionally, the stator and rotor are subjected to abrasive and corrosive drilling fluids and salts. Thus, replacement of worn stators is routine. To further reduce the abrasive effects of the downhole environment, the rotors are chrome plated. When the chrome is damaged along any part of the rotor body it becomes susceptible to failure from corrosion and the entire rotor body must be rechromed which is costly.
Multilobed motors provide more torque but at a lower rotational speed. Thus, it is desirable to produce the large amounts of torque possible with the long motor and multilobed motors, while avoiding the manufacturing, handling, transportation and costly maintenance problems associated with long motor sections.
One way of simulating a long motor is to connect together several shorter motors in series, as described in Tschirky, U.S. Pat. No. 3,999,901. However, as each rotor is designed to eccentrically orbit within its own stator, difficulty in connecting the multiple motors together has resulted in limited success for this arrangement. Tschirky discloses a method for connecting the rotors in which the rotors are simply welded together. Welding the rotors together effectively eliminates the benefits of having separate motors. Another method described in Tschirky utilizes a pair of universal joints. The universal joints transfer torque between the rotors while permitting the rotors to eccentrically orbit within their respective stators, but out of phase with one another. While a universal joint may provide a working connection, it suffers from certain problems. The harsh environment of operating in the drilling fluid causes the universal joints to corrode and wear quickly. Additionally, the out of phase rotor operation causes a slight longitudinal movement that results in power transfer inefficiencies, instability and above normal wear on the stator.
Thus, it would be desirable to have a sturdy rotor-to-rotor coupling that allows two positive displacement motors to be releasably connected in series for increased torque but without the wear joints or moving bearing assemblies common in universal joints. It is also desirable to provide a sturdy rotor-to-rotor coupling to minimize the wear on the stators, and improve overall stability of the downhole motor arrangement.